product of average aerial photography daylight, camera filter trans- 
mission, and sensitivity of the film to be used in practice. Since 
the spectral responses of films used in aerial photography differ 
significantly, it is not possible to recommend one universal or 
standard spectral response. 
  
    
   
    
    
     
   
    
  
    
  
   
   
   
  
    
   
   
   
   
   
   
   
  
   
  
  
The spectral response of the film in practical aerial photography. 
shall be taken as its spectral response to an equal energy spectrum 
multiplied by the spectral energy distribution in "Sensitometric 
Average Daylight," and further modified by whatever filter is to 
be used with the lens. (Ref 2) 
6.3 Focus Settings C e 
The determinations shall be made in a series of focal planes 
adjacent to the plane of nominal best focus. 
One plane shall be chosen as the reference plane. Preferably 
the actual distance of the reference plane from some reference 
point on the lens mount shall be stated. 
Sufficient determinations shall be made in different planes 
to enable selection of a plane giving optimum performance over 
the field according to the requirements of the user, and to indicate 
the tolerance in depth of focus. 
6.4 Range of Spatial Frequencies 
Measurements shall be taken at least over the range 5-50 lines/mm 
and to higher spatial frequencies if indicated by the performance 
and use of the lens. Measurements shall be taken at sufficient 
frequencies to reveal the shape of the MTF curve. é o 
6.5 Azimuth Settings 
To check for lack of symmetry (departure of optical axis from 
mechanical axis), OTF measurements shall be made off axis, both 
sides of axis, at one angular position (e.g. 0.7 field) with the 
lens rotated to at least three azimuthal direction around its mechanical 
axis. Any significant asymmetry shall be reported. The main measure- 
ments shall be made in the azimuth showing no (or least) asymmetry. 
6.6 Target Orientation 
The target or targets shall be oriented in two directions 
at right angles so that the MTF is measured,in effect, for radial 
lines and for lines at right angles to the radial direction. The 
report shall state the values obtained for both directions under 
each measuring condition.